One of the most critical problems confronting designers of high-intensity internal magnetic field sources has been accessing the internal field without distorting it. More specifically, those skilled in the art know that drilling an access port in the shell of a magnetic structure that produces an internal field, such as a magic sphere, can significantly distort the uniformity of that internal field.
Heretofore, several permanent magnet structures have been designed to provide distortion free access to an internal uniform magnetic field. The following references, which are hereby incorporated herein, show many types of magnetic structures that can be utilized for such purposes: (1) Pat. No. 5,396,209 entitled "Light-Weight Magnetic Field Sources Having Distortion-Free Access Ports," issued Mar. 7, 1995, to Leupold; (2) Pat. No. 5,216,401, entitled "Magnetic Field Sources Having Non-Distorting Access Ports," issued Jun. 1, 1993, to Leupold; (3) Pat. No 4,837,542, entitled "Hollow Substantially Hemispherical Permanent Magnet High-Field Flux Source For Producing A Uniform High Field," issued Jun. 6, 1989, to Leupold.
These reference patents describe different techniques for altering the magnetization of permanent magnet shells which generate a uniform field in an internal cavity such that a portion of the shell could be removed to access the internal field, from outside the entire structure, without disrupting the uniformity of that internal field. As described in the patents referenced above, such shell altering techniques can be performed on shells having many different sizes and shapes. Of particular significance are those structures having a spherical shell of a cylindrical shell. When the shell of such spherical and cylindrical structures are altered so that access ports can be drilled through the magnetic poles or the equator of the shell, the resultant structures are called magic spheres and magic rings, respectively. See, Pat. No. 5,216,401, and Pat. No. 4,837,542.
Briefly, a magic sphere is a magnetic structure having a spherically-shaped permanent magnet shell wherein the shell has a predetermined thickness and a magnetization that varies as a function of the polar angle around the shell. Similarly, a magic ring has a ring-shaped permanent magnet shell that has a predetermined thickness and magnetization that varies as a function of the polar angle around the shell. As disclosed, the magnitude of the internal magnetic field directly depends on the thickness and the magnetization of the shell. Thus, depending on the size and magnetization of the shell material, an artisan could design a structure proving an internal field over a wide range of magnetization, wherein that field could be access from outside the shell through ports drilled in predetermined locations in the material.
It has been shown, however, that although these shell-altering techniques provide distortion-free access to the internal field, the resultant internal field is substantially weaker than the that of an un-altered structure of the same size. Accordingly, the inventor herein developed a method of increasing the strength of the internal field without distorting the uniformity of the internal field. See Pat. No. 5,382,936, entitled "Field Augmented Permanent Magnet Structures," issued to Leupold et al., on Jan. 17, 1995, incorporated herein by reference.
As shown, by placing permanent magnets and/or passive ferromagnets in predetermined locations within the internal chamber of the shell, a higher internal field could be achieved with less shell material. As a result, these field-augmented permanent magnet structures became highly desirable to those having a premium on space.
These structures as well as all other permanent magnet high field sources, however, presented several problems to the user. More specifically, due to the small and narrow shape of the access ports through the shell of the structure, access to the internal working field must be from outside the entire structure itself. In addition, if the internal cavity contained field-augmenting magnets or passive ferromagnets, the size of the internal cavity is also very small and narrow. As a result, the structure was not very useful for most applications wherein space is at a premium and wherein a large internal work space is needed to house the uniform internal field.